Oldest species with a backbone dates back more than 500 million years

By William On Jul 8, 2022

The oldest known relative of all vertebrates on Earth swam the oceans 518 million years ago, a new study reveals.

Researchers in China have analysed fossils of yunnanozoans, an extinct soft-bodied organism that lived during the Cambrian Period of our planet’s history.

The fossils, found in the Yunnan Province, China, show the creature is the Earth’s oldest-known ‘stem vertebrate’ – a vertebrate that’s extinct, but very closely related to living vertebrates.

Yunnanozoans were very simple fish-like organisms that lived underwater, but they had ‘basket-like’ skeletons similar to today’s vertebrates.

They are also thought to have been deuterostomes – meaning their anus formed before their mouth during embryonic development.

Artistic reconstruction of the yunnanozoan underwater shows ‘basket-like’ skeletons similar to today’s vertebrates

Vertebrates, including fish, amphibians, reptiles, birds, mammals, and humans, share unique features, such as a backbone and a skull, while invertebrates are animals without backbones.

WHAT WERE THE YUNNANOZOANS?

Yunnanozoans are an extinct fish-like soft-bodied organism that lived 518 million years ago.

Yunnanozoans are the oldest-known ‘stem vertebrates’ – vertebrates that are extinct, but very closely related to living vertebrates.

They’re thought of as deuterostomes – their anus formed before their mouth during embryonic development.

Fossils of the yunnanozoan have been uncovered from China’s Yunnan Province

The new study has been conducted by experts at the Nanjing Institute of Geology and Paleontology and the Nanjing University in China’s Jiangsu province.

‘Yunnanozoans are Cambrian animals with a taxonomic position that has long been debated,’ the team say in their paper.

‘Our phylogenetic analysis provides further support that yunnanozoans are stem vertebrates.’

Scientists have long puzzled over the gap in the fossil record that would explain the evolution of invertebrates to vertebrates.

The evolutionary process that prompted invertebrates to develop backbones, and what those earliest vertebrates looked like, has been a mystery for centuries.

As scientists have studied how vertebrates evolved, a key focus has been the pharyngeal arches – paired structures that grow on either side of the future head and neck of the developing embryo and fuse at the middle.

Mammal embryos have five pairs of these pharyngeal arches. As a mammal embryo grows, the pharyngeal arches produce parts of the face and neck, such as the muscles, bone, and connective tissue.

The authors say pharyngeal arches are a ‘key innovation’ that likely contributed to the evolution of the jaws and braincase of vertebrates.

It’s thought the pharyngeal arch evolved from a ‘rod’ of unjointed cartilage in vertebrate ancestors, such as the chordate amphioxus, a small ‘fish-like’ organism and close invertebrate relative of the vertebrates.

In an effort to better understand the role of the pharyngeal arch in ancient vertebrates, the research team studied the fossils of the soft-bodied yunnanozoans found in the Yunnan Province, China

But whether such anatomy actually existed in the ancient ancestors has not been known for certain.

In an effort to better understand the role of the pharyngeal arch in ancient vertebrates, the research team studied the fossils of 127 yunnanozoan specimens.

The specimens have well-preserved carbonaceous residues that allowed the team to conduct detailed analyses, using microscopy, spectrometry and other methods.

Results confirmed that yunnanozoans have cellular cartilages in the pharynx, a feature considered specific to vertebrates, suggesting they are stem vertebrates.

During their study, the team also observed that all of the seven pharyngeal arches in the yunnanozoan fossils are similar to each other.

All the arches have bamboo-like segments and filaments. Neighbouring arches are all connected by horizontal rods and the top and bottom, forming a basket.

A basket-like pharyngeal skeleton is a feature found today in living jawless fishes, such as lampreys and hagfishes.

Pharyngeal arches are paired structures that grow on either side of the future head and neck of the developing embryo and fuse at the middle. Depicted here is a rat embryo with pharyngeal arches

‘Two types of pharyngeal skeletons – the basket-like and isolated types – occur in the Cambrian and living vertebrates,’ said study author Tian Qingyi.

‘This implies that the form of pharyngeal skeletons has a more complex early evolutionary history than previously thought.’

Researchers describe the yunnanozoans as ‘controversial’, because their classification has been debated for around three decades.

But the new anatomical observations support the evolutionary placement of yunnanozoans at the very bottom of the vertebrate tree of life.

Their new study has been published in the journal Science.

DIFFERENCES BETWEEN AN INVERTEBRATE AND A VERTEBRATE

A vertebrate is an animal with a back bone or cartilage covered spinal cord. The term stems from the word vertebrae, the bones that make up the spine.

Animals that do not have backbones or cartilage covered spinal cords are called invertebrates. Vertebrates include birds, fish, amphibians, reptiles, and mammals.

Approximately 97 per cent of animals on our planet are invertebrates, or animals with no backbone, even though when we think of ‘animals’ we tend to think of vertebrates.

Invertebrates are sometimes mistakenly thought of as primitive because of their lack of developed organs.

Their simple internal systems include respiratory systems such as gills or trachea and they often use an open circulatory system to pump their blood.

As invertebrates lack an internal skeletal structure, they sometimes have an external skeleton that protects their soft bodies called an ‘exoskeleton’.

In general, invertebrate success often comes from their ability to reproduce extremely quickly, unlike many vertebrates who take years to become fully grown.

Vertebrate animals, on the other hand, have a spine that develops from a notochord they possess as an embryo.

They also have defined internal systems like complex respiratory structures, a closed circulatory system and sensory organs that build the nervous system.

Vertebrates tend to be larger than invertebrates, thanks to their backbone, which allows their bodies to grow larger and move faster than many invertebrates.

Source: Ellen Eisenbeis/Butterfly Pavilion